1. Field of the Invention
The present invention relates to a Josephson junction device having weak link of artificial grain boundary and a process for preparing the Josephson junction device, and more specifically to a Josephson junction device of an oxide superconductor, of which the barrier of the weak link is constituted of a grain boundary of two single crystal oxide superconductor regions and a process for preparing the Josephson junction device.
2. Description of Related Art
A Josephson junction device which is one of superconducting devices can be realized in various structures. Among the various structures, the most preferable structure in practice is a stacked junction realized by a thin non-superconductor layer sandwiched between a pair of superconductors. However, a point contact type junction, a Dayem bridge type junction and a variable thickness bridge type junction which are composed of a pair of superconductor regions which are weakly linked to each other also exhibit Josephson effect. In general, these Josephson junctions have fine structures in which the superconductor and/or non-superconductor are composed of thin films.
In order to realize a stacked type junction by using an oxide superconductor, a first oxide superconductor thin film, a non-superconductor thin film and a second oxide superconductor thin film are stacked on a substrate in the named order.
In the above mentioned stacked type junction, an insulator MgO etc., a semiconductor Si etc., and a metal Au etc. are used for the non-superconductor layers so that each superconducting junction has different properties for each applications.
The thickness of the non-superconductor layer of the stacked type junction is determined by the coherence length of the superconductor. In general, the thickness of the non-superconductor layer of the stacked type junction must be within a few times of the coherence length of the superconductor. Since oxide superconductor materials have a very short coherence length, therefore, a thickness of a non-superconductor layer must be about a few nanometers.
However, the superconductor layers and the non-superconductor layer of the stacked type junction must be of high crystallinity for favorable junction properties, which are composed of single crystals or composed of polycrystals which are orientated in almost same direction. It is difficult to stack an extremely thin and high crystalline non-superconductor layer on an oxide superconductor layer. Additionally, it is very difficult to stack a high crystalline oxide superconductor layer on the non-superconductor layer stacked on an oxide superconductor layer. Though the stacked structure including a first oxide superconductor layer, a non-superconductor layer and a second oxide superconductor layer is realized, the interfaces between the oxide superconductor layers and the non-superconductor layer are not in good condition so that the stacked type junction does not function in good order.
In order to manufacture a point contact type junction, a Dayem bridge type junction and a variable thickness bridge type junction by using oxide superconductor, very fine processings which realize a weak link of a pair of superconductor are necessary. It is very difficult to conduct a fine processing with good repeatability.
The point contact type junction has been formed of two oxide superconductor thin films which are in contact with each other in a extremely small area which constitutes the weak link of the Josephson junction.
The Dayem bridge type junction has been formed of a constant thickness oxide superconductor thin film which is formed on a substrate and which is patterned in a plan view, so that a superconductor thin film region having a greatly narrow width is formed between a pair of superconductor thin film regions having a sufficient width. In other words, the pair of superconductor thin film regions having a sufficient width are coupled to each other by the superconductor thin film region having the greatly narrow width. Namely, a weak link of the Josephson junction in the superconductor thin film is formed at the greatly narrow width region.
On the other hand, the variable thickness bridge type junction has been formed of an oxide superconductor thin film of a sufficient thickness which is formed on a substrate and which is partially etched or thinned in a thickness direction, so that a thinned oxide superconductor thin film portion is formed between a pair of superconductor thin film portions having the sufficient thickness. In other words, the pair of superconductor thin film portions having the sufficient thickness are coupled to each other by the thinned oxide superconductor thin film portion. Accordingly, a weak link of the Josephson junction is formed at the reduced thickness portion of the oxide superconductor thin film.
As would be understood from the above, a characteristics of the Josephson device has a close relation to the contact area of the superconductor thin film in the point contact type Josephson device, the width of the superconductor thin film region having the extremely narrow width in the Dayem bridge type Josephson device, and to the thickness of the thinned oxide superconductor thin film portion in the variable thickness bridge type Josephson device, both of which form the weak link of the Josephson junction. Therefore, in order to obtain a desired characteristics with a good repeatability, a high precision on a sub-micron level of the processing such as the etching is required.
The Dayem bridge type Josephson device can be said to be more preferable than the variable thickness bridge type Josephson device, since the Dayem bridge type Josephson device has a relatively planer surface, which is preferred in a integrated circuit. However, in order to form the weak link in the Dayem bridge type Josephson device, it is required to pattern an oxide superconductor thin film having the thickness on the order of 0.5 .mu.m to 1.0 .mu.m into a width of not greater than 0.2 .mu.m. However, it is very difficult to conduct this fine patterning with good repeatability.
On the other hand, in the variable thickness bridge type Josephson device, the very fine pattering is not required in order to form the weak link. However, it is very difficult to uniformly control the remaining thickness of the thinned portion forming the weak link. In addition, the variable thickness bridge type Josephson device cannot have a planer surface by nature. This is not preferable to the integrated circuit application.
Therefore, in the prior art, it is almost impossible to manufacture a superconducting device for example a de SQUID (superconducting quantum interference device) which has multiple homogeneous Josephson junctions utilizing an oxide superconductor.
In order to resolve the above mentioned problems, researches have been conducted to manufacture a Josephson junction device taking account of the characteristics advantage intrinsic to the oxide superconductor, permitting it to avoid the fine processing of the oxide superconductor.
The superconducting characteristics of the oxide superconductor considerably varies, depending on the crystalline direction. Particularly, the oxide superconductor has a large critical current density in the direction perpendicular to the c-axes of its crystals. Thus, if the oxide superconductors having crystalline directions different from each other are joined together, a grain boundary at the interface becomes a barrier of the weak link so that a Josephson junction is formed. A Josephson junction device utilizing this Josephson junction is called artificial grain boundary type Josephson junction device. A Josephson junction device of this type can be manufactured without the fine processing as mentioned above.
In order to manufacture the artificial grain boundary type Josephson junction device, in a prior art, the same two substrates were joined so as to form a substrate of which the crystalline directions of each sides were different from each other. Then, an oxide superconductor thin film was deposited thereon so as to form an oxide superconductor thin film having different crystalline directions in both sides. However, according to the process, it was difficult to join the substrates having different crystalline directions and particularly the junction interface was not sharp so that the artificial barrier forming the weak link of the Josephson junction device could not be formed sharply.
In another prior art, a specific buffer layer was formed on a portion of the substrate and an oxide superconductor thin film was deposited thereon. A portion of the oxide superconductor thin film which grew on the buffer layer had a crystalline orientation different from that of the other portion of the oxide superconductor thin film which grew directly on the substrate. In this process, there are very few buffer layer materials which have these characteristics and which does not affect the oxide superconductor at all. In addition, buffer layer patterning process might affect the principal surface of the substrate on which the oxide superconductor thin film is deposited.